I seriously doubt whether the consultant doctor and his general practitioner wife imagine for one second that they are in a power struggle with their window cleaner.
You will never get the respect you desire from people like that, you are taking this far too personally. They don't care about you. They wanted £3 off their bill. It didn't work. I really don't understand why you let this get to you.
I have had guys work for me who hated to tell people "I am a window cleaner". They had similar issues with customers, it had a lot more to do with them and their insecurities than it did with the customer.
Doff your cap, say 'thank you muchly kind sir' and get on with it. No one give a toss but you. I don't mean to sound harsh but that is how I see it.